Catalytic hydroxylation of phenol

ABSTRACT

Hydroxylation of phenol is prone to the production of tarry by-products. Selective hydroxylation of phenol can be obtained by reacting a limited amount of hydrogen peroxide with phenol in solution in a compatible organic solvent and in the presence of a catalyst that is at least partly soluble in the reaction medium and is the salt of a heteropolyacid of general formula: i) Q 3  PMo m  W 12-m  O 40  or ii) Q 3+v  PM n  V v  O 40 , in which Q represents a compatible organic cation, m is zero or an integer less than 6, M represents molybdenum or tungsten, v is an integer which is up to 3, and n is an integer such that n+v=12. A preferred organic cation comprises cetyl pyridinium. Selectivity towards catechol is particularly observed employing heteropolyacid salts in which m=0 in formula i) and when n=11 and M=tungsten in formula ii) and towards hydroquinone when n=11 and M=molybdenum in formula ii). Preferably the reaction medium comprises acetonitrile.

The present invention relates to a process for the hydroxylation ofphenol.

Phenol is a readily available raw material which can be hydroxylatedusing aqueous hydrogen peroxide and a catalyst to produce dihydricphenols, and particularly mixtures containing hydroquinone and catechol.However, two factors should be taken into account when seeking tohydroxylate phenol. First, the introduction of a second hydroxylsubstituent onto the aromatic nucleus tends to activate the moleculetowards further reaction and this leads to the formation of a mixture ofunwanted tarry by-products. Self-evidently it would be desirable tohydroxylate selectively, i.e. favour dihydric phenol formation comparedwith tarry by-product formation. Secondly, hydroquinone and catecholtend to be used for different purposes. For example, hydroquinone is aphotographic developer and catechol is an intermediate in the productionof industrial anti-oxidants. Consequently, it remains an aim ofproducers to obtain one or other product selectively.

A commercial process has been developed for hydroxylating phenol basedupon catalysed hydrogen peroxide which tends to produce mixturescontaining a major fraction of catechol, but additionally a minor,significant fraction of hydroquinone, typically in a mole ratio of about3:1. The proportion of tarry by-products has been controlled by limitingvery strictly to the use of very low mole ratios of hydrogen peroxide tophenol, but inevitably this restricts the extent of conversion of thephenol and hence the space yield of the plant.

European Patent Application, Publication No 0 338 666A, inventors MShimizu et al, describes a process in which a tri-alkyl substitutedphenol is converted to the corresponding tri-alkyl benzoquinone usinghydrogen peroxide and a heteropolyacid catalyst. The inventors Shimizuet al have applied a similar process to a wider range ofalkyl-substituted phenol substrates and described the results obtainedin a paper entitled "A convenient synthesis of alkyl-substitutedp-benzoquinones from phenols by a H₂ O₂ /heteropolyacid system",published in Tetrahedron Letters, vol 30, No 4, pp471-474. These twopublications teach that alkyl substituted phenols are converted throughto the corresponding benzoquinone rather than selectively to anhydroxylated product. The only teaching regarding phenol itself was thatit gave only a trace amount of p-benzoquinone. Accordingly, neitherpublication provides any clear teaching on how to hydroxylate phenolselectively.

It is an objective of the present invention to identify more exactly theconditions in which any one or more of the disadvantages in theaforementioned commercial process can be ameliorated or overcome.

According to the present invention, there is provided a process for thehydroxylation of phenol in which phenol is dispersed in a compatibleorganic reaction medium and brought into contact with hydrogen peroxidein the presence of a catalyst characterised in that the catalyst, whichis at least partly soluble in the reaction medium, is selected fromsalts of heteropolyacids of general formula

i) Q₃ PMo_(m) W.sub.(12-m) O₄₀ or

ii) Q_(3+v) PM_(n) V_(v) O₄₀

in which Q represents a compatible organic cation, m is zero or aninteger less than 6, M represents molybdenum or tungsten, v is aninteger which is up to 3, and n is an integer such that n+v=12 and nomore than a limited amount of hydrogen peroxide is employed.

In British Patent Application no 9103323.3, filed on 16th Feb. 1991 fromwhich priority is claimed, the use was described of certain catalystsfor selective phenol hydroxylation obeying the general formula:

    Q.sub.3 PMo.sub.n W.sub.(12-n) O.sub.40

in which Q represents a compatible organic cation and n is an integerwhich is less than or greater than 6. Herein, such catalysts when n isless than 6 are represented by general formula i, in which it will beobserved that m has been substituted for n. Subsequent analysis of thecatalyst tested in support of the formula in which n was greater than 6showed that the anion in HPS11 had the formula PMo₁₁ VO₄₀, i.e.contained vanadium as a hetero metal atom rather than tungsten. The useof such catalysts in which n has a high value together with relatedcatalysts was described in British Patent Application No 9200929.9 tothe same applicants from which priority is also claimed. Such catalystsare described herein with reference to formula ii.

By selecting the salt of the selected heteropolyacids, it has been foundthat hydroxylation of phenol can occur to a significantly greater extentthan when the same heteropolyacid in acidic form was employed, thelatter in extreme cases being unable to promote any significant amountof hydroxylation. Alternatively, the salt can promote a significantlyimproved selectivity of conversion of the phenol to a particulardihydric phenol, compared with use of the acid form of catalyst underotherwise the same reaction conditions.

By selecting the composition of the heteropolyacid catalyst inaccordance with the description provided herein, it has been foundpossible, particularly in conjunction with the selection of the organicreaction medium, to observe not only selective hydroxylation of phenolto dihydric phenols, but also hydroxylation having a substantial degreeof selectivity towards a specific dihydric phenol, being eitherhydroquinone or catechol.

The organic cation Q is desirably an onium cation and particularly anammonium or phosphonium cation. It is preferable to select a cation thatis commensurate in size with the heteropolyacid anion, to at least areasonable extent. In accordance therewith, it is particularly suitablefor the cation to contain at least 8 carbon atoms and preferably fromabout 15 to about 30 carbons. The carbons may be distributed evenly asfor example in tetra ethyl or tetrabutyl ammonium or one or two of thealkyl substituents can contain a disproportionate number of carbons, asin a long chain substituent of from e.g. 9 to 18 carbons with theremaining alkyl substituents being short chain, such as ethyl of methyl.Two or more of the alkyl substituents can combine to form with thehereto atom, e.g. nitrogen, a heterocycle, such as pyridinium. Anespecially convenient range of onium cations comprises alkyl pyridiniumcations in which the alkyl contains from 12 to 18 linear carbons, suchas cetyl pyridinium.

The choice of heteropolyacid anion is of importance in directinghydroxylation selectively towards either hydroquinone or catechol. Sucha choice can be made on the basis of analysis of the products obtainedin a small scale trial of each heteropolyacid anion employing conditionsaccording to the present invention. It has been found in the presentinvestigations that in trials employing a heteropolyacid anion in whichn=11 in formula ii, when the metal was molybdenum, some selectivitytowards the production of hydroquinone was observed and towards catecholwhen the metal was tungsten. Likewise, when m=0 in formula i), i.e.tungsten but no molybdenum was present, some selectivity towards theproduction of hydroquinone was also observed. In comparison trials whichemployed a salt of a heteropolyacid in which n=6 and M=tungsten, i.e. aprocess not according to the present invention, no selectivity towardsdihydric phenol production was observed, let alone selectivity towards aparticular dihydric phenol.

It will be understood that the catalyst is described herein in terms ofthe heteropolyacid salt that is introduced into the reaction mixture,and that during the reaction period, a fraction of the heteropolyacidmay become transformed in situ to species different from thatintroduced. The present invention specifically includes the use of anyactive species which is so derived in situ, even if that species doesnot accord with the general formulae given hereinabove.

It is desirable to choose a suitable temperature at which to carry outthe hydroxylation reaction. The temperature selection takes into accountthe need to dissolve phenol and at least a proportion of theheteropolyacid catalyst in the reaction medium. In practice it has beenfound that a temperature in the vicinity of at least 40° C. isadvantageous. The upper limit comprises the reflux temperature formedium, which naturally varies depending upon the chemical nature ofthat medium. In practice, the reaction is often effected at atemperature of from 50° to 80° C. In many operational embodiments, theamount of catalyst is such that it is completely dissolved when thereaction is conducted at a temperature of at least 50° C.

A suitable solvent in which the reaction can be carried out has beenfound to include low molecular weight aliphatic carboxylic acids,including in particular acetic acid. A preferred class of solventscomprises low molecular weight nitriles and in particular acetonitrile.Such a class of solvent is preferred in that it includes the bestsolvent of those tested as regards optimising the selectivity of thereaction towards, for example, hydroquinone as predominant product.Mixtures of the two aforementioned classes of solvent can be employed.

One especially desirable combination of catalyst, solvent and reactiontemperature comprises the use of a salt in which the cation is a C10 toC18 pyridinium ion and the anion contains tungsten or especiallymolybdenum in a very high ratio to vanadium, such as about 11:1, thesolvent comprises acetonitrile as the major proportion and thetemperature is from 50° to 80° C.

One factor of importance in conducting this reaction appears to comprisethe amount or proportion of water present in the reaction medium. It isinevitable that some water will be present not only because the latteris a reaction product and a decomposition product of hydrogen peroxidebut also because water is the diluent in current commercial grades ofhydrogen peroxide. In general, the amounts of water introduced orgenerated by the use of commercial concentrated hydrogen peroxide can betolerated, such as those containing at least 35% and especially from 50to 70% w/w H₂ O₂.

It has been found that it is desirable to avoid introducing or employingan excessive amount of water, particularly when using a molybdenum-richheteropolyacid salt as catalyst. For such catalysts, it is advantageousto avoid the introduction of further water as part of the reactionmedium, though some advantage can be retained even if water comprises upto 2/3rds approximately by volume of the solvents employed. It has beenobserved when using a salt of a heteropolyacid that is molybdenum-rich(e.g. n=11), that introduction of additional water beyond that presentin concentrated hydrogen peroxide promotes catechol production. Thisthereby impairs somewhat the selectivity of hydroquinone productformation, although overall the selectivity towards the total dihydricphenol production remains.

When employing a salt of a tungsten-rich heteropolyacid, such as thevanadium-free heteropolyacids of formula i), e.g. those in which m=0,the presence of additional water has likewise been observed to boostcatechol formation. However, for such tungsten-rich salts, it may evenbe beneficial, in that in a number of trials it has been seen toreinforce the tendency of the tungsten-rich catalyst to produce asignificant proportion of catechol, thereby improving selectivity. Thus,when using such a salt, it is convenient to employ a significantfraction of water in the solvent; such a fraction is from about 1/3rd toabout 4/5ths water, though selected such that the solvent mixtureremains as a single phase.

The best results according to the present invention have been obtainedwhen phenol had been converted in a proportion of from about 5 to 20%and preferably from about 10 to about 15%. Accordingly, in one mostdesirable aspect of the invention, the reagents are preferably selectedso as to achieve such a conversion or the reaction quenched whenmonitoring of the reagents and products indicates that a conversion inthat range has been attained.

It is desirable, in processes according to the present invention, toemploy at least 0.01 mmole and up to 100 mmole of catalyst salt per moleof phenol, and preferably from 0.1 to 5 mmole per mole of phenol. Insome processes, it has been convenient to employ phenol significantly ina weight excess to catalyst, such as at least 2.5:1 and significantyields of dihydric phenols have been obtained using a phenol:catalystweight ratio selected in the range of from 5:1 to 15:1.

The concentration of phenol is normally selected within the range of 0.1to 2 moles per liter, taking into account such factors as thetemperature and composition of the reaction medium.

One other variable comprises the mole ratio of hydrogen peroxide tophenol. In processes according to the present invention, no more than alimited amount of hydrogen peroxide is employed, with the specificintention of reducing the extent to which over-oxidation of phenol mayoccur. Generally, it is desirable to restrict the ratio of hydrogenperoxide:phenol to about 4:1 or less. It will be recognised that thisamount is substantially less than the amount of hydrogen peroxideadvocated in the patent application and paper by Shimizu referred tohereinbefore. Significant yields of dihydric phenols have been obtainedwhen the mole ratio of peroxide consumed to phenol present initiallyfell in the range of from about 1:1 to about 2:1. Unreacted phenol maybe recovered from the reaction medium and employed in a subsequentreaction. It will be recognised that the lower preferred limit forhydrogen peroxide:phenol will depend at least partly upon the extent towhich a user is willing to recycle unreacted phenol. It is preferablefor at least 5% phenol to be reacted in each cycle.

In practice, the present invention is carried out for reasons ofconvenience and safety by forming a solution of phenol and the catalystin the selected reaction medium at the selected reaction temperature andthereafter introducing the selected amount of hydrogen peroxide solutionover a significant introduction period, preferably gradually. Aconvenient peroxide introduction period is often chosen from the rangeof 15 to 75 minutes. Thereafter, it is desirable for the reactionmixture to be maintained with agitation at the reaction temperature fora further reaction period such that the overall period of introductionand reaction is from about 2 to 7 hours.

Having described the present invention in a general way, specificembodiments thereof are described hereinafter in greater detail by wayof example only.

In the Comparisons and Examples, the catalyst HPA0 was availablecommercially and catalysts HPS0, HPS6, HPA11, HPS11, HPS9, HPS10, HPWA11and HPWS11 were all made in accordance with the process disclosed by YMatoba et al in Synthetic Communications, 14, p865 (1984) forrespectively a heteropolyacid or salt thereof. They were characterisedas follows:

    ______________________________________                                        Catalyst  cation           anion                                              ______________________________________                                        HPA0      H                PW12O40                                            HPS0      cetyl pyridinium PW12O40                                            HPS6      "                PMo6W6O40                                          HPA11     H                PMo.sub.11 VO.sub.40                               HPS11     cetyl pyridinium PMo.sub.11 VO.sub.40                               HPS9      "                PMo.sub.9 V.sub.3 O.sub.40                         HPS10     "                PMo.sub.10 V.sub.2 O.sub.40                        HPWA11    H                PW.sub.11 VO.sub.40                                HPWS11    cetyl pyridinium PW.sub.11 VO.sub.40                                ______________________________________                                    

Although the catalysts HPS9 and HPS10 had the approximate stoichiometryindicated, it is believed that they may contain a mixture of anionsincluding the designated species and PMo₁₁ VO₄₀.

Examples using HPS0, HPS11, HPS9, HPS10 and HPWS11 are according to theinvention and processes using HPA0, HPS6 and HPA11 are included by wayof comparison.

COMPARISONS C1 TO C4

In these Comparisons, trials were conducted to determine if the catalystdescribed in the aforementioned European Patent Application listingShimizu et al as inventors would cause hydrogen peroxide to react withphenol at the reaction temperature and mole ratio of peroxide to phenolspecified therein to produce selectively a dihydric phenol product. Ineach of these Comparison trials, a three necked round bottom flask of 25ml capacity, equipped with a magnetic stirrer, thermometer, condenserand inlet tube for hydrogen peroxide was charged phenol (0.2 g, 2mmole), solvent (10 ml) and catalyst (HPA0 or HPS11). The contents ofthe flask were brought to a temperature of 30° C. in a water bath andhydrogen peroxide, (35% solution w/w, 40 mmole) was introduced graduallyinto the stirred mixture over a period of 45 minutes using a peristalticpump. The reaction was permitted to continue for a further 5 hours atthe same temperature. The reaction mixture was analysed by reverse phaseHPLC employing silica coated with ODS as the stationary phase and either2% acetic acid in acetonitrile or 2% aqueous acetic acid as the eluant.Residual hydrogen peroxide, if any, was determined by potassiumiodide/sodium thiosulphate titration.

The conditions and results are summarised in Table 1 below. In theseComparisons and subsequent Examples, "Product Yield" means the molarpercentage obtained of the specified dihydric phenol product, based onphenol introduced at the start of the reaction, "para" indicateshydroquinone, "ortho" indicates catechol and "Conversion of phenol"means the molar percentage of phenol that has been consumed during thereaction, unless otherwise stated. A "-" indicates that a trace at mostwas produced or converted, depending on the context.

                  TABLE 1                                                         ______________________________________                                        Sol-             Mole Ratio Product Yield                                                                          Conversion                               vent Catalyst    phenol:H2O2                                                                              para ortho of phenol                              ______________________________________                                        C1   AcOH    HPA0    1:20     --   --    17.0                                 C2   MeCN    HPA0    1:20     --   --    --                                   C3   AcOH    HPS11   1:20     --   --    15.0                                 C4   MeCN    HPS11   1:20     --   --     6.8                                 ______________________________________                                    

From Table 1, Comparisons C1 and C2, it can be seen that no significantamounts of dihydric phenol were produced from phenol using the processconditions and catalyst of Shimizu et al, viz a low reactiontemperature, substantially greater than a limited amount of hydrogenperoxide and using HPA0. Some phenol was converted when the solvent wasacetic acid, but when the acetonitrile was selected as solvent, nosignificant conversion of phenol occurred. This indicates that catalystHPA0, which was in the acidic state was unable to promote any reactionbetween hydrogen peroxide and phenol in acetonitrile, let alone acontrolled reaction to generate a dihydric phenol. From Comparisons C3and C4, it can be seen that under the same process conditions, a relatedcatalyst, introduced as a salt, HPS11, was likewise unable to generate asignificant amount of a dihydric phenol in either solvent, although aminor conversion of phenol was observed.

COMPARISONS C5, C8, C9, C13 AND C15; EXAMPLES 6, 7, 10 TO 12 AND 14

In these Examples and Comparisons, trials were conducted to see if theinvention catalysts and the compounds in acid form could promotedihydric phenol formation under conditions different from thosedescribed by Shimizu.

In each of these Examples and Comparisons, a 100 ml three necked roundbottomed flask, equipped with a magnetic stirrer, condenser, thermometerand peroxide inlet tube was charged with phenol (2.0 g, 21.3 mole),solvent (40 ml) and catalyst (0.2 g). The flask contents were thenheated to 70° C. using a water bath and maintained at that temperaturethroughout the reaction period. Hydrogen peroxide was introducedcontinuously over a period of 45 minutes to attain the specified moleratio to phenol and the reaction mixture was permitted to react for afurther 5 hours. The resultant reaction mixture was analysed by themethod described for Comparisons C1/2. Reaction variables and theresults are summarised in Table 2 below.

                  TABLE 2                                                         ______________________________________                                                                               Con-                                                                          version                                Sol-               Mole Ratio Product Yield                                                                          of                                     vent Catalyst      phenol:H.sub.2 O.sub.2                                                                   para ortho phenol                               ______________________________________                                        C5   AcOH     HPA0     1:2      1.6  --    40.1                               Ex6  AcOH     HPS0     1:2      --   5.8   39.3                               Ex7  AcOH     HPS11    1:2      5.6  --    22.8                               C8   MeCN     HPA0     1:2      --   --    --                                 C9   MeCN     HPS6     1:2      --   --    5.3                                Ex10 MeCN     HPS0     1:2      --   1.7   11.6                               Ex11 MeCN     HPS11    1:2      11.7 --    12.2                               Ex12 MeCN/    HPS0     1:2      --   4.1   22.3                                    H.sub.2 O                                                                C13  MeCN     HPA11    1:2       0.65                                                                              --    8.8                                Ex14 MeCN     HPWS11   1:2      --   3.5   6.1                                C15  MeCN     HPWA11   1:2      --   --    22.5                               ______________________________________                                    

From Table 2, it can be seen that under the different conditions of thepresent invention, hydroxylation of phenol was promoted with thecatalysts that were in salt form, viz HPS0, HPS11 and HPWS11, except forHPS6, but to a much lesser or nil extent using the same catalysts inhydrogen form, viz HPA0, HPA11 and HPWA11. When heteropolyacid saltHPS11 was employed, i.e. that which was rich in molybdenum, improvedhydroxylation of phenol to hydroquinone (1,4-dihydroxybenzene) wasobserved. When heteropolyacid salt HPWS11 or HPS0 was employed, i.e.that which was rich in tungsten, selective hydroxylation of phenol tocatechol (1,2-dihydroxybenzene) was observed.

Ex11 demonstrated a particularly effective combination of processconditions and catalyst, in that the process resulted not only in a highselectivity of conversion of phenol to dihydric phenols, but alsoindicated a high selectivity to the production of one specific dihydricphenol, hydroquinone.

EXAMPLES 16 TO 19

In these Examples, a number of further trials were conducted in whichExample 11 was repeated, but employing a number of variations which aresummarised in Table 3 below, together with the results of the trials. InEx16, the volume ratio of MeCN to H₂ O was 15:25.

                  TABLE 3                                                         ______________________________________                                             Phenol            Catalyst                                                                              Mole Ratio                                                                             Reaction                                   (g)     Solvent   (g)     phenol:H.sub.2 O.sub.2                                                                 Temp °C.                       ______________________________________                                        Ex16 3.0     MeCN      HPS11 0.4                                                                             1:2      60                                    Ex17 3.0     MeCN      HPS11 0.2                                                                             1:1      60                                    Ex18 2.0     MeCN      HPS11 0.4                                                                             1:1      80                                    Ex19 3.0     MeCN/     HPS11 0.2                                                                             1:2      80                                                 H.sub.2 O                                                        ______________________________________                                        Reaction       Yield Product                                                                              Conversion                                                Period (hr)    para   ortho of phenol                                 ______________________________________                                        Ex16    2              8.9    1.0   15.1                                      Ex17    4              1.8    1.7   6.3                                       Ex18    4              6.3    1.6   21.9                                      Ex19    4              10.0   1.7   11.6                                      ______________________________________                                    

From Table 3, it can be seen that the catalyst/solvent system was ableto function in a range of process conditions and still yield significantproportions of dihydric phenol products.

EXAMPLES 20 AND 21

In these Examples, the process of Example 11 was repeated, but usingrespectively catalyst HPS9 and HPS10. The results are summarised inTable 4.

                  TABLE 4                                                         ______________________________________                                        Sol-             Mole Ratio Product Yield                                                                          Selectivity                              vent Catalyst    phenol:H2O2                                                                              para ortho to phenol                              ______________________________________                                        Ex20 MeCN    HPS9    1:2      2.1  --    15.8                                 Ex21 MeCN    HPS10   1:2      5.5  --    18.9                                 ______________________________________                                    

The results in Table 4 confirm that the catalysts containing a highratio of molybdenum to vanadium direct the reaction towards hydroquinonerather than catechol.

We claim:
 1. In a process for the hydroxylation of phenol whichcomprises reacting phenol with hydrogen peroxide in a compatible organicreaction medium in the presence of a catalyst to produce a dihydricphenol, the improvement wherein the catalyst, which is at least partlysoluble in the reaction medium, is selected from salts ofheteropolyacids of the formulai) Q₃ PMo_(m) W_(12-m) O₄₀, or ii) Q_(3+v)PM_(n) V_(v) O₄₀ in which Q represents a compatible organic cation, m iszero or an integer less than 6, M represents molybdenum or tungsten, vis an integer which is up to 3, and n is an integer such that n+v=12,and wherein not more than a limited amount of hydrogen peroxide isemployed.
 2. A process according to claim 1 wherein n=1 in formula ii)for the heteropolyacid.
 3. A process according to claim 1 wherein thecompatible cation Q is an onium cation containing at least 8 carbonatoms.
 4. A process according to claim 3 characterised in that the oniumcation comprises an alkyl pyridinium cation, preferably in which thealkyl group contains from 12 to 18 linear carbon atoms.
 5. A processaccording to claim 1 wherein the reaction medium comprises a lowmolecular weight aliphatic carboxylic acid.
 6. A process according toclaim 1 wherein the reaction medium comprises a low molecular weightaliphatic nitrile.
 7. A process according to claim 1 wherein the amountof hydrogen peroxide introduced into the reaction mixture is in a moleratio to phenol selected within the range of about 4:1 or less.
 8. Aprocess according to claim 7 wherein the amount of hydrogen peroxideintroduced into the reaction mixture is in a mole ratio to phenolselected in the range of 1:1 to 2:1.
 9. A process according to claim 1wherein the concentration of hydrogen peroxide introduced into thereaction mixture is no lower than 35%, including any water alreadypresent in the reaction mixture, when the catalyst comprises the salt ofa tungsten-rich heteropolyacid.
 10. A process according to claim 1wherein the reaction medium comprises from 1/3rd to about 4/5th parts byvolume of water when the catalyst comprises the salt of amolybdenum-rich heteropolyacid.
 11. A process according to claim 1wherein the reaction is terminated when from 10% to 15% of the phenolhas reacted.
 12. A process according to claim 1 wherein the reactiontemperature is selected in the range of from 50° to 80° C.
 13. In aprocess for the hydroxylation of phenol which comprises reacting phenolwith hydrogen peroxide in a compatible organic reaction medium in thepresence of a catalyst to produce a dihydric phenol, the improvementwherein the catalyst, which is at least partly soluble in the reactionmedium, is selected from salts of heteropolyacids of the formulai) Q₃PW₁₂ O₄₀, or ii) Q_(3+v) PM_(n) V_(v) O₄₀ in which Q represents acompatible organic cation, m is zero or an integer less than 6, Mrepresents molybdenum or tungsten, v is an integer which is up to 3, andn is an integer such that n+v=12, and wherein not more than a limitedamount of hydrogen peroxide is employed.
 14. A process according toclaim 5 wherein acid comprises acetic acid.
 15. A process according toclaim 6 wherein said nitrile comprises acrylonitrile.